Former Virginia Lt. Gov. Don Beyer handily won the Democratic primary Tuesday night for a U.S. House seat representing a district in the Northern Virginia suburbs of Washington, defeating six challengers in a race.

With 67.5% of precincts reporting, Mr. Beyer, who runs a well-known car dealership in the region, had won 44.8% of the vote. He has been widely considered the race’s frontrunner.

Mr. Beyer’s victory became clear early in the night, and the Democrat declared victory just past 8 p.m. Addressing his supporters, he said, “Tonight is the culmination of the hard work, the heartfelt values, and the shared ideas of many, many, many good people.”

Mr. Beyer is virtually guaranteed a win this November and will succeed Democratic Rep. Jim Moran, who announced earlier this year that he would be retiring after 12 terms. The district swings heavily Democratic and has elected President Barack Obama with about two-thirds of the vote in both 2008 and 2012.

The district’s primary stood out for its candidates’ fervent embrace of the Obama agenda, a campaign tactic that stands in stark contrast to the strategy of Democrats elsewhere, who have largely sought to put distance between themselves and the president.

Mr. Beyer, 60 years old, was elected lieutenant governor in 1989 and served two terms. In 2008, he campaigned for Mr. Obama and later became ambassador to Switzerland and Liechtenstein. Afterward, he returned to his family’s auto dealership. He had vastly outraised every other candidate, pulling in $1.1 million through May 21.

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